Hot answers tagged

6

ISIS works quite differently than most of the protocols you know. In the original OSI networking, ISIS would make adjacencies with it's neighbors, and tell them about the adresses of any ES it knows. IS means intermediate system, so besically a router. ES means end system, so any host. ISIS has different logic with different levels of routing. The levels ...


5

L1 routers don't know anything about the inter-area topology, so they choose the nearest L2 router. R1 doesn't know about R6, the route is just "outside my area", so for R1 that's R2. R2 doesn't know about R6 but it knows that it's inside area 2. The shortest path to area 2 is over R3. R3 routes to R5 which does know about R6 but uses the shorter path over ...


5

I guess this is because there is no next-hop IP address for those two routes? Correct, but let's be clear, it's not that one is there and the router can't find it, it's because no next-hop IP address exists. Just because you're advertising and receiving IP TLVs (which Juniper does by default) on a valid adjacency doesn't mean that the underlying ...


4

What difference does it make? OSPF and ISIS behave very similarly in the sense that they both use methods to form neighborships/adjacencies with Hello packets (or PDU's) and then exchange topology information (LSA flooding), then their link state databases are constructed, and then the Dijkstra algorithm is run to generate a shortest-path tree and then the ...


4

Most IPv6 routing protocols use the link-local address of the next hop. Remember that the only purpose of the next hop address is to determine the MAC address of that next hop (on ethernet at least. Other media use similar mechanisms). Except for this lookup the address itself is never used. So it doesn't matter that you only see the link-local address. It ...


4

By default redistribution of IBGP into an IGP is disabled. To redistribute IBGP routes into an IGP, you need the command bgp redistribute-internal. Configuring this on B will inject BGP routes into ISIS. router bgp 65000 bgp redistribute-internal More info here:https://www.cisco.com/c/en/us/td/docs/routers/xr12000/software/xr12k_r3-9/routing/...


3

It seems that I was correct. The interface ge-2/1/0.0 will have both level 1 and level 2 disabled. I have just seen this question on the Junos Genius APP for Android.


3

I mean what does traffic engineering has to do with wide-metric style for IS-IS? What extra information wide-metric starts to carry when enabled , which is required by TE and not supported by narrow metric style of IS-IS. In order for IS-IS to support traffic engineering, the "old-style" link-state PDU TLVs (Type-2 IS-Reachability and Type-128 IP-...


3

From the RFC1142 OSI IS-IS Intra-domain Routing Protocol 3.6 Additional Definitions ... 3.6.2 Neighbour: An adjacent system reachable by tra versal of a single subnetwork by a PDU. 3.6.3 Adjacency: A portion of the local routeing information which pertains to the reachability of a single neighbour ES or IS over a single circuit. Adjacencies ...


3

IPv4 and IPv6 are two incompatible protocols. In order to forward your IPv6 data across an IPv4 network, you have to use some sort of tunneling mechanism, where the IPv6 packet is encapsulated in an IPv4 packet. This is because IPv4 devices do not understand IPv6. IS-IS or any other routing protocol, has nothing to do with this process. The function of ...


3

clear concept is that cisco packet tracer does not support isis routing protocol


3

That will fail. MTU is setting the L2 MTU while IP MTU is setting the L3 MTU. The CLNS MTU determines both the maximum size of LSP plus the hello size when padding. If you tried to send a 2000 byte LSP over a 1500 MTU L2 link, it will get dropped


3

Reference to this answer is not easy to find in any vendor/training materials, but lies in RFC1195: Level 2 routers include in their level 2 LSPs a list of all [IP address, subnet mask, metric] combinations reachable in their area. In general, this information may be determined from the level 1 LSPs from all routers in the area. If we ignore resource ...


2

It is typically the IP address tied to the physical interface, but not always. JunOS from Juniper, for example, sends the Router-ID which can be a loopback and not tied to a physical address. Source: https://kb.juniper.net/InfoCenter/index?page=content&id=KB25145&actp=search


2

In essence - take a look at RFC 1195, section 5.1: IP Interface Address -- the IP address(es) of the interface corresponding to the SNPA over which this PDU is to be transmitted. Why IP needs to be there? Remember that IS-IS is L2 protocol and L3 information (IPv4 or IPv6 address information) is overlay information for IS-IS. It's not needed for IS-IS ...


2

You should check FIB on your switch if you find there two records it means that the switch will load-balance. The command is: display fib x.x.x.x 32 verbose In the output you'll also see whether switch will use mpls switching for this FEC and what LSP will it pick. Fields LspFwdFlag and LspToken indicate this.


2

OSPF and ISIS use the maximum-paths command to specify the maximum number of equal cost paths the SPF algorithm will take into account. By default this is set at 4, so by default up to 4 equal cost paths will be considered to each destination. You can view the maximum paths for each protocol using the show ip protocols command: Switch#sh ip protocols *** ...


2

Route metrics are used by routing protocols to determine the best paths through your network. The actual metric isn't used to determine the path unless the Administrative Distance is the same between two or more paths. Cisco uses a default Administrative Distance of 0 for directly connected paths. If you want to learn more about Administrative Distance, here'...


2

I just labbed it in VIRL. For the PC subnet, I get a metric of 50331642 on Router D: Routing entry for 10.0.0.4/30 Known via "isis 1", distance 115, metric 50331642, type level-2 Installed Feb 28 17:46:19.166 for 00:00:07 Routing Descriptor Blocks 10.0.0.17, from 192.168.0.2, via GigabitEthernet0/0/0/0 Route metric is 50331642 No ...


2

I am testing with Cisco routers. I have two routers with 2 parallel links between them (instead of 3 in your diagram), all of the links are point to point. It does seem that all of the adjacencies will be active, when I run show clns neighbors or show isis neighbors I see two entries, one for each link show isis neighbors: System Id Type Interface ...


2

IS-IS is a routing protocol. It can forward on information about what direction to send traffic for a number of protocols, including IPv4 and IPv6. The key point there is that it's information about what direction traffic should be forwarded but it's not the means by which the traffic is actually forwarded. The simplest analogy is that of roads, road ...


2

In theory, just like for OSPF, there is no limit. There will be a practical limit, based on the routing protocol traffic generated, bandwidth, and router resources. Some people have networks that work with 1000 routers for either OSPF or IS-IS, but that would be rare, and it is probably a network that needs to be redesigned. I actually worked on one, and it ...


2

Some implementations set the ATT bit automatically and provide you a command or knob under a configuration stanza to disable it. i.e on Juniper: ignore-attached-bit Ignore the attached bit on IS-IS Level 1 routers. Configuring this statement enables the routing device to ignore the attached bit on incoming Level 1 link-state PDUs. If the attached bit ...


1

While IS-IS and OSPF work very similarly on the routing level (link-state, Dijkstra), likely the most important difference is that OSPF sits on top of IP (L3) while IS-IS sits on top of the data link layer (L2). Accordingly, IS-IS requires L2 connectivity between peers which isn't always available (e.g. when L3 tunneling is used with VPN or similar). ...


1

All OSPF and IS-IS interfaces have a cost, which is a routing metric that is used in the link-state calculation. Routes with lower total path metrics are preferred over those with higher path metrics. Unlike OSPF, in which the link metric is calculated automatically based on bandwidth, there is no automatic calculation for IS-IS. All IS-IS links use a metric ...


1

This could get a little heavy so brace yourself. From a routing perspective, Understand the concepts of VRFs ( virtual routing forwarding tables ). Generally, a lot of ISPs use MPLS to route traffic across their core, and this would mean using either L3VPN or L2VPN for providing a circuit between your sites. Each customer could come into the ISP core on a ...


1

A Level 1/Level 2 system sets the attached bit in the Level 1 PDUs that it generates into a Level 1 area to indicate that it is a Level 2-attached backbone router and that it can be used to reach prefixes outside the Level 1 area. Level 1 routers create a default route for interarea prefixes, which points to the closest (in terms of metrics) Level1/Level 2-...


1

IS-IS doesn't help you with crossing IPv4 territory for IPv6, you'll need to use a tunnel.


1

Just to clarify some terms, which may actually muddy the water a bit. @mellowd's answer is a great short answer. CLNS is the OSI connectionless network service definition: it specifies the interface that the service user (transport layer) uses to communicate with its peer transport layers. CLNP is the OSI protocol that provides that service, and is rarely ...


1

Try to issue command #ip router isis edit: Cisco Packet Tracer doesn't support ISIS. Cisco Packet Tracer Supported Protocols: Application • FTP , SMTP, POP3, HTTP, TFTP, Telnet, SSH, DNS, DHCP, NTP, SNMP, AAA, ISR VOIP, SCCP config and calls ISR command support, Call Manager Express Transport • TCP and UDP, TCP Nagle Algorithm & IP Fragmentation, ...


Only top voted, non community-wiki answers of a minimum length are eligible