Question

Problem 3 Consider the network shown in Figure 3. Suppose that AS1 and AS2 use OSPF for their intra-AS routing, and AS3 and AS4 use IGRP for intra-AS routing. Initially, no physical link exists between AS4 and the other ASes. Suppose all the intra-AS links have equal cost. AS 1 1b AS 4 4b 4a 4c 4d AS 3 3b 1a 1c AS 2 2b 11d 12 2a 2c 112d 12 3a 3d 3c X Figure 3. Example of BGP. a) Give the routing protocol used to learn about prefix x at each of the following routers: 3a, 2c, 2d, 1a. (hint: select among OSPF, IGRP, eBGP, IBGP) b) Which interface ($i_1$ or $i_2$) will router 2d use to reach x? Why? c) Now suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose router 2d learns that x is reachable via both AS3 and AS1, with identical local preference. Which interface will 2d use to reach x? Why? d) Again suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose router 1d learns that x is reachable via both AS2 and AS4, with identical local preference. Which interface will 1d use to reach x? Why?

          Problem 3
Consider the network shown in Figure 3. Suppose that AS1 and AS2 use OSPF for their intra-AS routing,
and AS3 and AS4 use IGRP for intra-AS routing. Initially, no physical link exists between AS4 and the
other ASes. Suppose all the intra-AS links have equal cost.
AS 1
1b
AS 4
4b
4a
4c
4d
AS 3
3b
1a
1c
AS 2
2b
11d 12
2a
2c
112d 12
3a
3d
3c
X
Figure 3. Example of BGP.
a) Give the routing protocol used to learn about prefix x at each of the following routers: 3a, 2c,
2d, 1a. (hint: select among OSPF, IGRP, eBGP, IBGP)
b) Which interface ($i_1$ or $i_2$) will router 2d use to reach x? Why?
c) Now suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose
router 2d learns that x is reachable via both AS3 and AS1, with identical local preference. Which
interface will 2d use to reach x? Why?
d) Again suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose
router 1d learns that x is reachable via both AS2 and AS4, with identical local preference. Which
interface will 1d use to reach x? Why?

Problem 3
Consider the network shown in Figure 3. Suppose that AS1 and AS2 use OSPF for their intra-AS routing,
and AS3 and AS4 use IGRP for intra-AS routing. Initially, no physical link exists between AS4 and the
other ASes. Suppose all the intra-AS links have equal cost.
AS 1
1b
AS 4
4b
4a
4c
4d
AS 3
3b
1a
1c
AS 2
2b
11d 12
2a
2c
112d 12
3a
3d
3c
X
Figure 3. Example of BGP.
a) Give the routing protocol used to learn about prefix x at each of the following routers: 3a, 2c,
2d, 1a. (hint: select among OSPF, IGRP, eBGP, IBGP)
b) Which interface (i1 or i2) will router 2d use to reach x? Why?
c) Now suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose
router 2d learns that x is reachable via both AS3 and AS1, with identical local preference. Which
interface will 2d use to reach x? Why?
d) Again suppose that links are added between AS4 and AS1/AS3 as shown in Figure 3. Suppose
router 1d learns that x is reachable via both AS2 and AS4, with identical local preference. Which
interface will 1d use to reach x? Why?

Added by Elena K.

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