Loopbacks or no loopbacks with EIGRP

[CiscoBrownBelt]

Is it best to create loopbacks for networks that utilize EIGRP for WAN routing? I know some places do and some don't.

[chrihussey · Accepted solution]

Loopbacks really don't come too much into play with EIGRP. For example, OSPF uses the loopback as its router ID. Not so in EIGRP.

However, using loopbacks is a good way to manage a router. This way it has a single logical  IP that shouldn't go down. Without them you have to communicate or connect to the router on a physical interface which could go down.

[CiscoBrownBelt]

Awesome thanks so much! Question though, for my situation, if all IP configs are under 1 sole interface, if this interface were to go down I pretty much can't access the device anyway so the loopback would be beneficial if I had at least 2 physical connections to the device correct?

[chrihussey · Accepted solution]

Yes you are correct. Generally routers have multiple interfaces and with that comes redundancy. However, if there is only one interface, it is what it is. Regardless, if there is a chance additional inetrfaces or routed connections could occur, having the loopbacks in place would only help.

[Peter Paluch · Accepted solution]

Hi chrihussey,

In fact, EIGRP does have and use Router IDs. It is perhaps not so publicly known but they are used, and their relative obscurity gives rise to some unexpected issues - most importantly, a router will ignore any route that is marked with its own RID. We've seen situations when a router has been migrated to another one with the original one just being readdressed but not reloaded, and that caused two routers in the network to have the same EIGRP RID, resulting in both routers mutually ignoring their own advertised routes.

EIGRP exchanges routing and metric information using various TLV (Type-Length-Value) versions. The first TLV version was 1.2 which was used up to and including IOS 12.4T; this TLV version only included the RID with external (redistributed) routes. Newer TLV versions are TLV 2.0 (multi-protocol TLVs used with EIGRP Wide Metrics), TLV 3.0 (multi-topology TLVs, deprecated), and TLV 4.0 (used in SAF, Service Advertisement Framework). Both TLV 2.0, 3.0, and 4.0 attach EIGRP RIDs both to internal and external routes.

It is unfortunately somewhat difficult to determine what TLV version does a particular router use, but if you execute show eigrp plugins and see the "eigrp-release" plugin of the version 8.0 and higher, this EIGRP supports TLV 1.2 and TLV 2.0. With neighbors of a router, you can pull out the show ip eigrp neighbor detail which shows the TLV version advertised by the particular neighbor:

Router# show ip eigrp neighbors detail 
EIGRP-IPv4 Neighbors for AS(1)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
0   10.0.12.2               Et2/0                    11 00:00:21    1  3000  0  1
   Version 18.0/2.0, Retrans: 1, Retries: 0
   Topology-ids from peer - 0 

Router# show ip eigrp neighbors detail 
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   10.0.12.2               Fa0/0             12 00:00:07   33   200  0  3
   Version 12.4/1.2, Retrans: 1, Retries: 0

The EIGRP RID can be always seen in the heading of the show ip eigrp topology:

Router# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(10.255.255.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 10.255.255.1/32, 1 successors, FD is 128256
        via Connected, Loopback0
P 10.0.12.0/24, 1 successors, FD is 281600
        via Connected, Ethernet2/0

Newer IOS versions also display this information in show eigrp protocols:

Router# show eigrp protocols
EIGRP-IPv4 Protocol for AS(1)
  Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  Soft SIA disabled
  NSF-aware route hold timer is 240
  Router-ID: 10.255.255.1
  Topology : 0 (base) 
    Active Timer: 3 min
    Distance: internal 90 external 170
    Maximum path: 4
    Maximum hopcount 100
    Maximum metric variance 1

I thought you might be interested to know.

Best regards,
Peter

[Peter Paluch · Accepted solution]

Hi,

If a loopback is created on a router, EIGRP uses the highest IP from among all loopbacks as its Router ID (RID). Older EIGRP implementations add the RID only to redistributed networks; newer EIGRP implementations mark every route, both internal and external, with the RID of the router that first injected the route into EIGRP. Also, every EIGRP router ignores any route that is marked with its own RID.

Thus, using loopbacks with EIGRP is a simple way of making sure that the EIGRP RIDs will be unique, and in addition, the loopbacks - when injected into EIGRP - can be used to represent all your routers in a very simple way; one "universal" loopback IP for the whole router.

To sum it up, having loopbacks with EIGRP is not mandatory, but certainly can be useful.

Best regards,
Peter