The "local route" and "connected route" on IOS and IOS-XR

The "local route" and "connected route" on IOS and IOS-XR 


1.

Cisco IOS-XR Local and Connected Routes

• If an IPv4 address is configured on an interface of the router with a mask of /32, or an IPv6 address is configured with a mask of /128, the host routes appears with the L flag. 

• They are known through local, but installed as connected routes. Hence, these routes are redistributed when redistribute connected is configured under the routing protocol.

2.

Cisco IOS Local and Connected Routes

• The local routes have the administrative distance of 0. This is the same adminstrative distance as connected routes. 

• However, when you configure redistributed connected under any routing process, the connected routes are redistributed, but the local routes are not. This behavior allows the networks to not require a large number of host routes, because the networks of the interfaces are advertised with their proper masks. These host routes are only needed on the router that owns the IP address in order to process packets destined to that IP address.

3. Other point:

• The output of IOS:

Here is an interface with one IPv4 and one IPv6 address:

interface Ethernet0/0
 ip address 10.1.1.1 255.255.255.252
 ipv6 address 2001:DB8::1/64

The IP addresses assigned to Ethernet0/0 are 10.1.1.1/30 for IPv4 and 2001:db8::1/64 for IPv6. Neither are host routes. A host route for IPv4 has the mask /32, and a host route for IPv6 has the mask /128.
For each IPv4 and IPv6 address, Cisco IOS installs host routes in the respective routing tables.

 R1#show ip route 
 Codes: L - local, C - connected, S - static, R - RIP,
        M - mobile, B - BGP
        D - EIGRP, EX - EIGRP external, O - OSPF,
        IA - OSPF inter area 
        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA
        external type 2
        E1 - OSPF external type 1, E2 - OSPF external type 2
        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1,
        L2 - IS-IS level-2
        ia - IS-IS inter area, * - candidate default, U -
        per-user static route
        o - ODR, P - periodic downloaded static route, H - NHRP
        + - replicated route, % - next hop override

 Gateway of last resort is not set

 C        10.1.1.0/30 is directly connected, Ethernet0/0
 L        10.1.1.1/32 is directly connected, Ethernet0/0

In the previous routing table, 10.1.1.1/32 is a local host route.

 R1#show ipv6 route
 IPv6 Routing Table - default - 3 entries
 Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
        B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2
        IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external
        ND - Neighbor Discovery
        O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
 C   2001:DB8::/64 [0/0]
      via Ethernet0/0, directly connected
 L   2001:DB8::1/128 [0/0]
      via Ethernet0/0, receive
 L   FF00::/8 [0/0]
      via Null0, receive

In the previous routing table, 2001:db8::1/128 is a local host route. The FF00::/8 route is also a local route, but this route is needed for multicast routing and hence is not covered in this document.

•  The output of IOS-XR:

Here is an example:

 RP/0/RP0/CPU0:router#show route 10.10.10.1/32

 Routing entry for 10.10.10.1/32
   Known via "local", distance 0, metric 0 (connected)
   Installed Jul 10 10:50:30.265 for 00:20:07
   Routing Descriptor Blocks
     directly connected, via Loopback0
       Route metric is 0
   No advertising protos.

 RP/0/RP0/CPU0:router#show route ipv6 2001:db8:2222::1/128

 Routing entry for 2001:db8:2222::1/128
   Known via "local", distance 0, metric 0 (connected)
   Installed Jul 10 10:53:05.745 for 00:16:51
   Routing Descriptor Blocks
     directly connected, via Loopback0
       Route metric is 0
   No advertising protos. 

The result is that the router can always install a CEF entry for the configured IP address if it only searches for the corresponding entry in the routing table. This also prevents a misconfiguration where a route with a longer mask than the connected entry would be learned from another router, which causes traffic that is destined for the local IP address to be misdirected to a remote router.


For more please check:

http://www.cisco.com/c/en/us/support/docs/ip/ip-routing/116264-technote-ios-00.html#anc6
or
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080c14371.shtml