Redistribution Questions

Question 1

Question 2

Explanation

Use the external keyword along with the redistribute command to redistribute OSPF external routes.

In order to use an prefix-list in a “match” statement, we have to use the command “match ip address prefix-list …”. The syntax of a “match” statement is as follows:

match ip address {access-list-number [access-list-number… | access-list-name…] | access-list-name [access-list-number…| access-list-name] | prefix-list prefix-list-name [prefix-list-name…]}

Reference: https://www.cisco.com/c/en/us/td/docs/ios/iproute_pi/command/reference/iri_book/iri_pi1.html

Question 3

Explanation

The first statement of the above route-map (route-map BGP-TO-OSPF deny 10) will prevent network 172.16.1.0/24 from being redistributed into OSPF.

Question 4

Question 5

Explanation

It seems there is a loop because of mutual redistributions among RIP, OSPF and EIGRP domains. So we should filter out the prefix 10.1.1.0/24 when redistributed from OSPF to EIGRP (the second redistribution point) to prevent routing loop.

Question 6

Explanation

Only the subnet that matches prefix-list OSPF-TAG-PRF-1 will be redistributed into OSPF (as indicated by “route-map OSPF-TAG-1 permit 10”). This subnet must match the prefix-list OSPF-TAG-PRF-1 so it must be 10.2.0.0/18 to 10.2.0.0/24. Only the subnet 10.2.2.0/24 matches this requirement.

Note: The prefix-list “OSPF-TAG-PRF” denies a route (deny 10.1.0.0/16 le 24) is used in a route-map deny clause (route-map OSPF-TAG-1 deny 5) so according to Cisco:

“If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.”

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/49111-route-map-bestp.html

Therefore in this case the first route-map clause will be ignored so we don’t need to care about it.

Question 7

Explanation

If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default.

You can redistribute both internal and external (type-1 & type-2) OSPF routes via this command: “Router(config-router)#redistribute ospf 1 match internal external 1 external 2”

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5242-bgp-ospf-redis.html

Question 8

Explanation

In this topology, we are doing mutual redistribution at multiple points (between OSPF and EIGRP on R3 & R4), which is a very common cause of network problems, especially routing loops so you should use route-map to prevent redistributed routes from redistributing again into the original domain.

In this question, route-map is also used for this purpose. In particular, the route-map “SET-TAG” is used to prevent any routes that have been redistributed into OSPF from redistributed again into EIGRP domain by tagging these routes with tag 1:

R3 route-map SET-TAG permit 10 set tag 1

These routes are prevented from redistributed again by route-map FILTER_TAG by denying any routes with tag 1 set:

R4 route-map FILTER-TAG deny 10 match tag 1