Here i have this topology in my company enter image description here

cores configure with VSS, As we can see single link connecting each router to each core which make this link single point of fail, so I need to add link between each router and the other core acting as something like ether channel between two switches to avoid this failure probability from my network , like the next topology enter image description here

i'm asking about if there is any link aggregation method on the router side (some thing like L3 port aggregation on switches )

NOTE my routers plateform is 2921 and run Version 15.2(4)M6a

  • It depends on the router. Port channeling is normally a layer-2 (switch) STP thing Most routers can't port channel. I know some more recent Cisco routers with newer IOS can channel ports.
    – Ron Maupin
    Sep 5, 2015 at 17:19
  • Cross-Stack EtherChannel configuration example cisco.com/c/en/us/support/docs/switches/… Is this what you mean ?
    – psniffer
    Sep 5, 2015 at 17:25
  • dear i'm not asking about etherchannel configurations from the Switch side i just ask about how you doing link aggregation from the router side
    – Gadeliow
    Sep 5, 2015 at 17:33
  • 1
    A layer-3 link between the switch and the router is usually a /30 or /31. It's best to define the layer-3 address directly on the port instead of an SVI. For Cisco switches, put "no switchport" on the interface, and configure the interface as you would on a router. Run a routing protocol between all the layer-3 devices, and, with redundant links, any one link down will be routed around.
    – Ron Maupin
    Sep 5, 2015 at 19:59
  • 1
    You said are only looking for redundancy instead of bandwidth aggregation. That's what routing protocols are for; if properly configured they route around broken links. You could just run OSPF on your layer-3 devices, and each would have a complete understanding of all the paths available to get from one place in the network to any other. This is a conventional way to connect a layer-3 core switch pair to upstream routers. Some Cisco router/IOS combinations have something like what you are requesting, but, often, the simple method is best and can be used with almost any router.
    – Ron Maupin
    Sep 5, 2015 at 20:35

3 Answers 3


Based on your comments, you can achieve link redundancy by installing a link from each physical core switch chassis to each router. Each link would be a point-to-point (usually /30 or /31) routed link. The core and the routers would need to run an IGP routing protocol. Your devices support OSPF, and it is easy to set up and include every layer-3 interface. When a link is disabled, OSPF will route around the broken link.

This is a simple, conventional network topology that is easy to support on most business-class networks, and it seems to provide what you are looking for.

  • i beleve it could solve the issue and in this case may i use loopback interface in VG to register it on the CUCM
    – Gadeliow
    Sep 6, 2015 at 8:00
  • 1
    Using loopbacks is the preferred method for address references that you want to be reachable via routing when an interface or link goes down.
    – Ron Maupin
    Sep 6, 2015 at 8:10
  • so what about IRP , i posted answer based on this feature
    – Gadeliow
    Sep 6, 2015 at 22:21
  • IRB is not widely used. Layer-3 convergence on a change to the network is an order of magnitude faster than layer-2 convergence. Your problem is a straightforward one which can be dealt with using a standard network topology and practices. I'm not sure why you would want to deviate from that. Your problem was solved a long time ago, and the solution is easy and proven.
    – Ron Maupin
    Sep 6, 2015 at 22:31

here i found very good solution for this case is to use IRB (Integrated Routing and Bridging) , refer to this very long discussion IRB why and how?

A BVI (Bridge Group Virtual Interface) is a routed interface that represents a set of interfaces that gets bridged. so we need to configure IRB from the router side and the core side and it will provide the redundancy needed

i found some examples for such configuration in the next link EXAMPLE and i try it on GNS3 and it is working properly


I'm not sure if link aggregation is possible on a router, but if all you need is redundancy, link aggregation is NOT the way to go. Link aggregation is for increasing the bandwidth of a logical connection, not for redundancy.

  • for the 1st part of your answer , link aggregation is possible on ISR routers refer to the next link link aggregation , for the second part of your answer i agree with you main target behind LAG is to increase the BW , but it can provide you redundancy as well in case of one cable down the other is up
    – Gadeliow
    Sep 6, 2015 at 8:15

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